As is known, in order to guide a missile along a trajectory, in particular if said missile has to be subjected to significant and sudden load factors, use is made of guidance systems having lateral nozzles which are provided on board the missile and can be supplied with gas from either a gas generator of the main rocket motor or a gas generator specially provided for this purpose.
Thus, this results in lateral gas jets which generate transverse propulsive forces capable of quickly and significantly altering the trajectory of the missile. It can be ensured that the lines of action of such transverse forces pass through the centre of gravity of the missile, or at least in the vicinity of this centre of gravity, and in this instance the missile is said to be direct-thrust-controlled, since the time taken to respond to the control is then particularly quick. However, this is not mandatory and the lines of action of said transverse forces may pass the axis of the missile at points other than the centre of gravity. Similarly to conventional aerodynamic motivators, said transverse forces, then, create torques allowing the missile to be attitude-controlled relative to the centre of gravity.
To alter the cross section of flow for the gases flowing through the lateral nozzles and thereby act on the trajectory of the missile, guidance systems also comprise movable obturating devices which are provided between the generator and the nozzles, and control the flow of the gases from the generator.
In known embodiments, with each nozzle there is associated an obturating device comprising an obturator connected to the nozzle, and a driver which controls the movement of the obturator so as to alter the cross section of flow for the gases flowing through the neck of the nozzle. Thus, depending on their position, these devices supply thrust in a given orientation (depending on the geometric axis of the nozzle concerned) and in a single sense (dictated by the nozzle outlet).
By way of reminder, obturating devices are mainly:                of the on/off type, using pulse width modulation (PWM) with pneumatic power actuation to modulate thrust. The pressure in the chamber(s) of a pneumatic cylinder is altered by means of valves controlled by on/off electromagnets (from 0 to maximum thrust in as little time as possible, limited by the dynamics of the electromagnets and of the actuating cylinder of the obturator). However, this simple operating principle in fact induces violent shocks (of several dozen g) and vibrations in the structures and equipment of the machine to be guided and additionally prevents precise thrust control; or        of the electromechanical type, using electric motors associated with a reduction gear and/or a device for converting helical movement to control the position of the obturating device. Given the stresses and pressures at play, this type of device with rectilinear “needle-like” movement does not allow high dynamic performances to be achieved with a reasonable mass and spatial requirement compared with the preceding pneumatic devices.        
These devices have drawbacks. The fact that they are each associated with a nozzle implies complex production and bulkiness. Operationally speaking, the rectilinear movement of an obturator to more or less close the neck of a nozzle does indeed act in a given orientation (yaw or pitch for example), but does so in the sense dictated by the nozzle. These devices also have the drawback of generating very large stresses for manoeuvring the obturator. Any balancing-out of the stresses to reduce the necessary power is complex, as is the adjustment and motorisation of a differential system. In addition, with an electromechanical solution, the rotation of the engine has to be converted into a rectilinear movement in an environment which is highly unfavourable to hot gases.
Moreover, FR 2 659 734 discloses a system for guiding a missile using lateral gaseous jets of which the device for obturating the nozzles, of which there are four that are diametrically opposed two by two, thus comprises four rotating valves having obturators which distribute the gas flow from a solid rocket motor into the four nozzles.
An adjustable thrust can thus be achieved in two perpendicular orientations. The valves are mechanically coupled two by two to ensure a constant flow rate in each pair of diametrically opposed nozzles (danger of the solid rocket motor exploding due to overpressure) and the obturators are actuated by the pistons of pneumatic cylinders which are powered by drawing gases from the rocket motor and are position-slaved. The slaving of the position of the obturators by hot-gas pneumatic cylinders allows excellent dynamic performances to be achieved on account of their very large power-to-weight ratio.
Although successfully operational, this system is nevertheless heavy, relatively complex and fiddly to adjust, particularly on account of the need to mechanically connect two by two the obturating devices, which in this case are the obturators of the valves that are each connected to two diametrically opposed nozzles so as to act in the two senses of a single orientation. It is also necessary to use refractory materials subjected to high pressures. In addition, upon ignition of the solid rocket motor, there is unavoidably a transitory moment of uncontrolled slaving due to the increase in pressure and to the random position of the various components of the device (obturator, control ball). By allowing coupling of the obturators of the valves, the mechanical connection is furthermore a source of considerable friction, which impairs the quality of the slaving and means that the power of the actuating cylinders has to be over-dimensioned.
WO 96/13694 also discloses a system for launching and orienting flying machines, which system is provided with pairs of opposed nozzles that can be controlled by obturating devices and are located in the rear portion of the flying machine to put the same, after its launch from the ground, on the trajectory, heading towards the target.